1. Field of the Invention
The present invention relates to a field control coat of polymer material having electrically-conductive additives for medium voltage fittings.
2. Description of the Prior Art
Numerous field-controlling elements for cable fittings are known. A plurality of fillers are thereby employed as principal additives that form the field coating element in combination with a suitable polymer matrix and, potentially, other additives. For example, such fillers are inorganic substances having a high dielectric constant such as titanium (IV) compounds (titanium dioxide, barium titanate), electrically-conductive additives such as silicon carbide, carbon black, metal powder, as well as metal oxides, and semimetal oxides (zinc oxide, aluminum oxide, boron oxide, iron (II, III) oxide). Approximately 70% iron (II, III) oxide is therefore often employed as a principal additive, this being embedded in a polymer matrix of butyl rubber. This product is applied as an inner coating in a heat-shrinkable tube. Such a combination of materials, however, exhibits disadvantages given long-term use, particularly after the appearance of high transient overvoltages. The requirements thereafter are frequently no longer met.
A sealing compound of the above-mentioned type is provided as a field-controlling element for electrical field control in medium voltage fittings, this sealing compound being utilized as an inner coating in a shrink hose which, due to its composition, is in the position to avoid a disadvantageous stressing at the cable end by excessively-high field strengths.
This field-controlling coating must meet the following requirements:
For the purpose of a coating to be simply implemented, the field control coat must be formulated as a solvent-containing compound. The field-controlling coating forms from the coating in the hose by evaporation of the solvent that preferably has a low boiling point or, respectively, a high evaporation rate for this purpose; PA1 At least at the shrinking temperature, the field-control coating must have or develop adequately-high stickiness in order to guarantee an adhesion to the cable surface. The activation temperature of the coating should lie at 90.degree.-100.degree. C.; PA1 During the shrinking process, the compound must have adequate plasticity with good flow properties, so that the formation of bubbles or cavities can be prevented and a form-fitted connection to the substrate surface can be guaranteed; PA1 The current-voltage characteristics should be non-linear with a steep rise of the curve at approximately 4 kV/cm, with the following guide lines PA1 In particular the field-control coating, as a component part of a fitting, must pass the electrical tests of DIN 57 278/VDE 0278 (for example, for a 20 kV system) as set forth below
______________________________________ Voltage (kV) Current (.mu.A) ______________________________________ 1 0.1 3-4 1-2 8 up to 60; ______________________________________
Nominal power frequency withstand voltage (one minute withstand voltage of 55 kV), PA2 DC test (96 kV over 30 minutes), PA2 Nominal withstand surge voltage test (125 kV, 10 surges each of both polarities), PA2 Long-term AC voltage tests given cyclical current load (126 heating cycles, dry), PA2 Partial discharge measurement (free of partial discharge at 24 kV (.ltoreq.1 pC), and PA2 Long-term tests (load change at 30 kV).